1. Field of the Invention
The present invention relates to a development apparatus used in an image forming apparatus such as an electrographic apparatus or an electrostatic recording apparatus.
2. Related Background Art
In the past, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Post-Examination Publication No. 42-23910 and Japanese Patent Post-Examination Publication No. 43-24748, various methods are well-known, such as electrophotography. In general, an electrostatic latent image is formed on a photosensitive member formed from photoconductive material by various means, and then the latent image is developed by using toner as a toner image, which is in turn transferred onto a transfer material, such as paper. The toner image transferred to the transfer material is fixed to the transfer material by heating or solvent vapor, thereby obtaining a copy (reproduction).
As a developing method, various methods in which an electrical latent image is visualized by using two-component developer mainly including toner and carrier are already known. For example, there are various developing methods such as a magnetic brush developing method as disclosed in U.S. Pat. No. 2,874,063, a powder cloud method, a fur brush developing method, a liquid developing method and the like.
The magnetic brush method and liquid developing method using the two-component developer have widely been put to practical use since a good image can be obtained relatively stably. However, since both methods utilize the developer obtained by mixing the toner and the carrier, they have disadvantages such as deterioration of the carrier, variation in mixing ratio between the toner and the carrier, complication of the apparatus, scattering of the toner and/or uneven streak due to the presence of the carrier.
In order to eliminate such disadvantages, various developing methods using one-component developer (one-component toner) consisting of toner alone have been proposed. For example, U.S. Pat. No. 3,909,258 discloses a developing method using magnetic toner having electrical conductivity. In this method, conductive magnetic toner is carried on a cylindrical developing sleeve having a magnet therein, and, development is effected by contacting the developing sleeve with the latent image. During the development, in a developing portion where the developing sleeve is opposed to a photosensitive member, a conductive path is formed between the surface of the photosensitive member and the surface of the developing sleeve by toner particles so that charges are directed from the developing sleeve to the toner particles through the conductive path, with the result that the toner particles are adhered to an imaged portion by a Coulomb force between the imaged portion of the latent image and the toner particles, thereby developing the latent image.
Although the developing method using the conductive magnetic toner can eliminate the problems regarding the conventional two-component developing methods, since the toner is conductive, it has a disadvantage that it is difficult to electrostatically transfer the developed image from the photosensitive member to an ultimate support member such as a plain paper.
In order to eliminate this disadvantage, a developing method using high-resistance magnetic toner capable of being transferred electrostatically is disclosed in Japanese Patent Application Laid-open No. 52-94140 as a developing method utilizing dielectric polarization of toner particles. However, this method has a disadvantage that adequate density of the developed image cannot be obtained because the developing speed is essentially low, and, thus, it is difficult to put this method to practical use.
As another method using high-resistance magnetic toner, a developing method in which toner particles are frictionally charged by friction between the toner particles and friction between the toner particle and the developing sleeve and the development is effected by contacting the charged toner particles with the photosensitive member is known. However, it has been pointed out that this method has a disadvantage that poor frictional charging may occur because of small number of contacts between the toner particles and a friction member or the toner particles are apt to be aggregated on the developing sleeve if the Coulomb force between the charged toner particles and the developing sleeve, and it is practically difficult to realize this method.
On the other hand, in Japanese Patent Application Laid-open No. 54-43036, a new developing method eliminating the above-mentioned disadvantages has been proposed. In this method, magnetic toner is coated on a developing sleeve as a very thin layer and is frictionally charged and then a latent image is developed under a magnetic field by approaching the toner layer to the latent image without contacting with the latter.
According to this method, by coating the magnetic toner as the very thin layer, the chance for contact between the magnetic toner and the developing sleeve is increased thereby to permit the frictional charges required for development to be applied to the toner.
FIG. 5 shows an example of a magnetic one-component developing apparatus. As shown in FIG. 5, the developing apparatus comprises a developing container 10 containing magnetic one-component toner as developer, which container including a developing. sleeve 1, a permanent magnet 1a, two large and small toner conveying members 4 and a magnetic blade 11. The developing sleeve 1 is formed from a non-magnetic member and is disposed within an opening portion opposed to a photosensitive drum 100 as an image bearing member for rotation in a direction shown by the arrow B2, and the permanent magnet 1a has a roller shape and is disposed within the developing sleeve in a non-rotation manner.
The magnetic toner in the developing container 10 is conveyed to the developing sleeve 1 by the conveying members 4 and is borne on the surface of the developing sleeve 1 by a magnetic force of the magnet 1a and then is conveyed to a developing portion opposed to the photosensitive drum 100 as the developing sleeve 1 is rotated. On the way of the conveyance, the toner is regulated by the magnetic blade 11 spaced apart from the developing sleeve 1 by a distance W, with the result that a thin toner layer is coated on the developing sleeve 1. The distance W is generally selected to be 100 xcexcm to 1 mm.
A thickness of the toner layer coated on the developing sleeve 1 is determined by a position of a cut line L shown in FIG. 7. According to the inventors"" investigation, it was found that performance of the magnet toner will be as follows when the magnetic toner passes between the developing sleeve 1 and the magnetic blade 11.
As shown in FIG. 6, when planes perpendicular to a straight line connecting between the developing sleeve 1 and the magnetic blade 11 are considered and it is assumed that a plane near the magnetic blade is S1 and a plane near the developing sleeve 1 is S2, since a width of the magnetic blade 11 is generally selected to be smaller than a width of the magnet 1a, when magnetic flux densities on the planes S1, S2 are considered, the magnetic flux density on the plane S1 becomes greater than that on the plane S2. Accordingly, the magnetic toner on the developing sleeve 1 is subjected to a magnetic force directed toward a direction shown by the arrows, i.e., toward the magnetic blade 11 between the developing sleeve 1 and the magnetic blade 11, with the result that, as shown in FIG. 7, the magnetic toner particles t are interconnected between the developing sleeve 1 and the magnetic blade 11 thereby to form chains (or ears) as shown by xe2x80x9cBxe2x80x9d.
Application of charges to the magnetic toner particles t is effected with respect to end (developing sleeve 1 side) toner particles t1 by contact between the end toner particles t1 in the chains B and the developing sleeve 1. Further, the charges are applied to the end toner particles t1 in the chains B, the toner particles t1 are subjected to a force directed toward the developing sleeve 1 due to a mirror reflection force and are also subjected to a conveying force directed toward a rotational direction of the developing sleeve 1 due to a frictional force between the toner particles and the developing sleeve 1.
Further, since there is some aggregating force between the toner particles t, the conveying force is transmitted to a toner particle t2 contacted with the toner particle t1 via the aggregating force (cohesive force). Similarly, the conveying force is transmitted to a toner particle t3 contacted with the toner particle t2 via the aggregating force.
However, the toner particles t between the developing sleeve 1 and the magnetic blade 11 are also subjected to the magnetic force directed toward the magnetic blade 11. Accordingly, at a point where the conveying force acting on the toner particles overcomes the magnetic force, i.e., at the cut line L shown in FIG. 7, the toner chains B are broken, and the toner remaining on the developing sleeve 1 is conveyed in the rotational direction of the developing sleeve 1.
Accordingly, when magnetic toner having high aggregation (cohesive degree) or magnetic toner requiring greater contacting number to obtain the required frictional charge amount is used, toner particles not contacted with the developing sleeve 1 and thus having poor charges will be conveyed to the developing portion, thereby causing a poor image due to poor charging in the development.
In order to solve this problem, the inventors have proposed a technique in which, as shown in FIG. 8, a regulating sleeve 5 having a non-rotatable magnet 5a therein is provided as a developer regulating member for a developing sleeve 1 and a magnetic pole S21 of the magnet 5a is opposed to a magnetic pole N11 of a magnet 1a in the developing sleeve 1 and the regulating sleeve 5 is rotated in a direction opposite to the rotational direction of the developing sleeve 1 at an opposed area between the regulating sleeve and the developing sleeve.
With this arrangement, at the toner regulating portion where the developing sleeve 1 is opposed to the regulating sleeve 5, the magnetic toner on the developing sleeve 1 receives a conveying force mainly depending upon the charged amount of toner and directing toward the rotational direction of the developing sleeve 1 and a conveying force mainly depending upon the magnetic force and directing toward the rotational direction of the regulating sleeve 5, so that only sufficiently charged toner can be remained and uniformly coated on the surface of the developing sleeve 1 and be conveyed to the developing portion opposed to the photosensitive drum 100.
However, in the developing apparatus as shown in FIG. 8, since the amount of toner coated on the developing sleeve 1 is about ⅓ to xc2xd of that in the developing apparatus shown in FIG. 5, in order to obtain the image density, which is the same as that in the developing apparatus of FIG. 5, a rotational speed of the developing sleeve must be increased.
In this case, since the developing apparatus of FIG. 8 has high developing efficiency, even if the toner coating amount is about a half of that in the developing apparatus of FIG. 5, a peripheral speed ratio of the developing sleeve with respect to the photosensitive drum (rotational speed of developing sleeve/rotational speed of photosensitive drum) is not required to be twice, and, in order to obtain the same density as that of an image under the condition of a sleeve peripheral speed ratio of 1.2 to 1.5 frequently set in the developing apparatus of FIG. 5, a sleeve peripheral speed ratio may be set to 2 to 2.5. However, the sleeve peripheral speed ratio must still be set to a high value.
Accordingly, in response to high speed image formation of recent electrophotographic apparatuses, when the photosensitive drum is rotated at a higher speed, the rotational speed of the developing sleeve is increased more and more, with the result that, when toner having small particle diameter or toner having less magnetism is used, it is difficult to coat the toner on the developing sleeve stably.
An object of the present invention is to provide a development apparatus in which a high quality image can be obtained.
Another object of the present invention is to provide a development apparatus in which a high speed operation of a developing sleeve can be suppressed even when image formation is effected at a high speed, and, even when toner having small particle diameter or toner having less magnetism is used, magnetic toner having good charging characteristics can be coated on a developing sleeve stably and be applied to development.
A further object of the present invention is to provide a development apparatus comprising a first developer carrying member for carrying developer and adapted to develop a latent image formed on an image bearing member with the developer at a first developing area, a second developer carrying member for carrying the developer and adapted to develop the latent image formed on the image bearing member with the developer at a second developing area, and a regulating member borne on the second developer carrying member and adapted to regulate a thickness of a layer of the developer, and wherein the second developer carrying member regulates a thickness of a layer of the developer carried on the first developer carrying member.
The other objects and features of the present invention will be apparent from the following detailed explanation of the invention referring to the accompanying drawings.